Because of the original abundance of comets
and other debris in the early Solar System, even Jupiter could not protect the Earth from
the early cometary bombardment. However, this cometary bombardment was necessary to bring
the water necessary for life to exist on the Earth. Collisions were so frequent on the
early Earth that is is estimated that at least 30 large collisions (1) occurred that were
energetic enough to vaporize the entirety of the Earth's oceans (in addition to much of
its crustal rock) (2). However, once life was present on the Earth, the effect of cometary
impact was devastating. It seems likely that the extinction of the dinosaurs was caused by
the impact of a large meteor or comet with the Earth 65 million years ago.

References

Chyba, C. and C. Sagan. 1992. Endogenous production, exogenous delivery and impact-shock
synthesis of organic molecules: an inventory for the origins of life. Nature355:
125-132

Sleep, N.H., K.J. Zahnle, J.F. Kasting, and H.J.
Morowitz. 1989. Annihilation of
ecosystems by large asteroid impacts on the early Earth. Nature342:
139-142.

Photo credits: This picture is
used by permission of Wally Pacholka. It won the 1997 photo of the year from Time magazine. Hale-Bopp "as seen from Mars" titled image taken
4/4/97 at 8 PM from Joshua
Tree Nat Park using 50mm lens on a tripod, at f/2, 30 second exposure using
Fuji 800. A flashlight was used for 4-5 seconds to illuminate the foreground
rocks. Mr. Pacholka is
a strong Christian believer and a friend of Reasons To Believe.
More comet photos by Mr. Pacholka.